cmd/asm: allow def/ref of func ABI when compiling runtime

cmd/asm: allow def/ref of func ABI when compiling runtime

Function symbols defined and referenced by assembly source currently
always default to ABI0; this patch adds preliminary support for
accepting an explicit ABI selector clause for func defs/refs. This
functionality is currently only enabled when compiling runtime-related
packages (runtime, syscall, reflect). Examples:

  TEXT ·DefinedAbi0Symbol<ABI0>(SB),NOSPLIT,$0
        RET

  TEXT ·DefinedAbi1Symbol<ABIInternal>(SB),NOSPLIT,$0
        CALL    ·AbiZerolSym<ABI0>(SB)
	...
        JMP     ·AbiInternalSym<ABIInternal>(SB)
        RET

Also included is a small change to the code in the compiler that reads
the symabis file emitted by the assembler.

New behavior is currently gated under GOEXPERIMENT=regabi.

Updates #27539, #40724.

Change-Id: Ia22221fe26df0fa002191cfb13bdfaaa38d7df38
Reviewed-on: https://go-review.googlesource.com/c/go/+/260477
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Than McIntosh <thanm@google.com>